restricting user from leaving the page when back button is pressed

[sarath_777]

hi,
when a user logs into a site and does enter data
say time sheet,etc .. and they should not be allowed
to exit the site when they press BACK button ..
instead they have to ALWAYS press LOGOFF button
to exit ..

If they press BACK button it should come back to
the same page.. is there an easier way to do this..?
thanks

sarath